I would definitely go widescreen - just becuase it's more natural to the human eye.
It's is also VERY hard to find very large (+30") LCD displays anymore. I also wouldn't get anything smaller than 40". Gets hard to see the words and stuff when you get below that.

I would recommend going as big as you can afford. Once you get above 50" they are way out there in price. I don't think anyone will complain that the screen is too big. If you don't have room and budget allows there is always the option of hanging a second set of flat panels about midway. You can find some great deals on Macmall.com right now. They have some 47" LCD flat panels for $639.99 refurbished. I have a friend who is a pastor of a small rural church. They just remodeled their sanctuary and installed new LCD flat panels instead of projection screens. They went with what looks to me like 42". They probably should have gone a lot larger. He didn't ask for my opinion and I didn't give it. He got a "deal" from a local audio/visual installer and was so proud of what he got I didn't have the heart to put in my two cents worth. It's no telling what they paid for the system. They'll find out soon enough.

Careful in your purchase of LCD's. Make sure you get LCD's with HD15 VGA inputs unless you've got a computer with HDMI outputs.
Maybe someone will chime in with size recommendations, but I've got to run.

Thanks everyone for the great info. After measuring, the farthest seat will be about 55 feet away. If I convert that to inches and divide by 8, that means I need an 82" monitor! Yikes! That can't be right? I think 60" is about as big as you can get these days, no?

Thanks for the info on connections. Will definitely keep that in mind.

Optimal height is determined by measuring the distance from the farthest seat to the screen and dividing by 8.

Width is determined by your aspect ration (16:9 or 4:3) with your height.

The diagonal measurement would be determined by the Pythagorean theorem.

If you are just going to do words, you can go smaller and make the words bigger and put less words per screen.

But if you're even considering video, the above is the general rule of thumb that I've heard.

To perhaps clarify, the furthest viewer to image height factor of 8 is for video images. Typical computer graphics images would use a factor of 6, critical images (CAD, full spreadsheets, etc.) a factor of 4. It all comes down to being able to distinguish what is on the screen and for text that means being able to discern individual characters, something determined greatly by the character height. Thus using larger fonts can increase the acceptable viewing distance for the same image size.

The reality is that most flat screens have a rather limited viewing area, a 60" diagonal, 16:9 screen is only about 29-1/2" high so the furthest viewer at an 8:1 ratio would be less than 20' away. Increase that to 10:1 and you get close to 25'. Go down to a 50" diagonal screen and that becomes a 24-1/2" high image and a little over 16' (8:1) or around 20' (10:1). For a 42" diagonal display it is about a 20-1/2" high image and a little under 14' (8:1) or 17' (10:1) for recommended viewing. Those are fairly limited viewing distances for most sanctuaries unless you can provide multiple screens. You can get larger flat panels, larger LCD screens are widely available in the commercial market, but the prices are much greater and practical for most church applications. Or you can use larger fonts, this works for many but that may not be a feasible approach if you are trying to display lyrics, also show videos and so on.

Most people can get to a consumer electronics store, so I always suggest doing so and looking at the flat panel displays from a distance that relates to your situation, that may give a good idea of what you can expect. But do be wary of using consumer products, check the warranty carefully as some warranties specifically exempt any commercial or industrial use, effectively not supporting anything other than residential use.

We have a similar viewing distance at our place, and we have 6x8 screens. Yeah, they're 4:3 rather than 16:9, but the 6-foot height is what translates to you. There have been times I've wished for 7.5x10s, but we have a stupid low ceiling that factored in to what's practical -- and except for rare occasions, the 6x8s work plenty fine for us. We do use relatively tall text to help with that too.